Instrument failure is a familiar issue on vintage bikes. Smiths speedometers and tachs, used on the majority of British bikes up through the late 1970s, are generally reliable. Yet with age, failure becomes increasingly common. Specialty vendors can restore many of them, and while an excellent option, it might not be the route to take with a daily rider vintage Norton or Triumph.
A major issue with vintage Smiths speedometers is the speedometer gear box, which drives the speedometer cable at the wheel. These are known to fail regularly. Recent pattern replacements haven’t been as durable as the originals, and originals haven’t been available for years. So what’s the option? For some riders, it might be a digital speedometer.
Going digital has a distinct advantage; no mechanical components to fail. Instead of a gear box, a stationary pickup triggered by a magnet attached to the spinning wheel sends a signal to a processor, which calculates speed. It’s a simple proposition, but it does mean wiring up a trigger, which, while small, detracts from the vintage proposition. Fortunately, as we discovered, you can keep vintage looks while gaining modern electronics.
Smiths is still in business, and following an employee buyout in 2010 launched a new line of replacement digital speedometers and tachometers, now available in the U.S. from Smiths Gauges USA. The company, run by British bike enthusiast Tom Kullen, has engineered a simple conversion for the Smiths speedo drive so it can be used to house a digital pickup. To the casual observer, it looks stock and original.
New digital Smiths at left, old speedometer at right.
We were intrigued by the conversion, so when the speedometer on Tech Q&A answer man Keith Fellenstein’s 2002 Royal Enfield Bullet failed, we looked into the new Smiths digital gauges.
Royal Enfield Bullet fans will know that Indian-built Bullets didn’t use a Smiths speedometer, so why pick Keith’s Bullet? Two reasons: First, the replacement digital units are the same size as the stock Bullet speedo, making them a good choice for RE owners as well as vintage Brit bike owners; second, since the Bullet’s speedo drive is a copy of the Smiths, we could perform the pickup conversion on a Smiths drive and use it on the Bullet — and almost everything we learned here applies to most 1960s to mid-1970s British bikes, plus Enfield Bullets up to about 2008.
This is not a technically challenging project. The only part we had to fabricate was the securing bracket for the speedometer, which may not be necessary in many applications. Disassembling and converting the speedometer drive is straightforward, as is the wiring. If you don’t want to do the conversion yourself, Kullen sells a complete kit that includes a refurbished and converted speedometer drive. Cost is $145. If your speedo drive works, you can opt for a direct-fit Hall-effect sensor that replaces the standard cable, simplifying the process.
A variety of speedos are available; we opted for the classic-looking chronometric-styled speedometer, which costs $379.50. The optional Hall-effect sensor is $45, and the standard Smiths speedometer gear box conversion kit is $89.50.
1. We used a donor speedo drive for the speedo drive conversion. To begin, punch out the four small rivets visible on the face plate using a punch slightly smaller than the rivets themselves.
2. The rivets secure a plate that holds the drive ring gear in place. Once the rivets are released the plate will fall out. Next, use a screwdriver to pry off the cover with felt seal keeping the ring gear in place. It removes fairly easily.
3. Secure the unit in a vise clamped onto the base of the threaded brass speedometer cable adapter. Use a brass punch or stake and drive the speedometer drive housing off the cable adapter. Again, it will remove fairly easily.
4. Once the housing is separated from the cable adapter, remove the cable drive gear from the housing. It should just fall out. The adapter and gear are not used in the conversion.
5. Our donor drive unit was used on the rear wheel of a Norton. To use it on the front wheel of our Bullet, we had to swap the brass plug from one end to the other. This is easily done by driving it out using a punch, then gently pressing or hammering it back in the other side.
6. Next, install the digital pickup in the speedometer drive body. The digital pickup has a stepped plastic body. Gently press the digital pickup into the drive body until the small end of the digital pickup is fully visible in the drive body.
7. Next, we removed the front wheel from our Royal Enfield and then removed the stock speedometer drive unit from the wheel hub. It’s held by a single 15/16-inch nut.
8. Here’s our Bullet’s front wheel with the stock drive unit removed and ready to install the reluctor for the electronic pickup. As it spins past the pickup, magnets in the reluctor generate a signal for the digital speedo. The seal in the reluctor comes into play on Norton, BSA and Triumph applications.
9. Next, apply a thin bead of RTV silicone to either the raised ridge on the hub or the flat inner surface of the reluctor. This is simply to help hold the reluctor in place.
10. Line up the two indexing tangs on the reluctor with the two slots on the raised lip of the hub and set it in place. Next, install the converted Smiths speedometer drive over the reluctor, then secure it with the stock 15/16-inch nut.
11. Now it was time to bolt the wheel back up to our Bullet. Here you can see the converted Smiths speedometer drive unit secured to the front wheel of our Royal Enfield. Make sure to position the unit where you want it before tightening down the axle caps.
12. As noted in the main story, if your original speedo drive is OK you can opt for a Hall-effect speedometer cable sensor that simply screws onto the original speedometer drive output. For the Enfield, it’s necessary to remove the square bushing from the cable drive and stack four 3/8-inch OD brass washers to properly space the drive.
13. Before removing the stock speedometer and speedometer cable, make sure the battery is disconnected. The stock speedometer is secured by a formed strap. The securing bolts for the Smiths unit are oriented 90 degrees off from the stock unit, so we had to make a new securing strap.
14. For a strap, we used a 5.5-inch long piece of 1-inch aluminum flat stock. The inside of the headlamp shell isn’t uniform front to back, so the strap has to have one long and one short leg. Drill the two 5/16in mounting holes 2.5 inches apart, as marked, then bend each end 90 degrees so you have a 1-inch leg and a 1.25-inch leg, as marked, and a 3.25-inch flat to secure the speedometer. The stub at far right was cut off.
15. Now you can install the Smiths speedometer unit using the strap you made to secure it in the headlamp shell. Use lock washers or nylock nuts and flat washers on the speedometer securing studs to ensure the strap stays tight.
16. The Smiths digital speedometer comes with a detailed and easy-to-follow wiring schematic. Following the schematic directions, wire up the Smiths digital speedometer.
17. As part of the wiring, you’ll need to wire in a switch to toggle the digital speedometer functions. Conveniently, our Enfield’s starter was removed long ago, so we used the stock electric starter button as the toggle switch for the speedometer.
18. Hook up the battery, turn on the ignition and perform the setup functions described in the directions that come with the speedo; you should be done. We installed neutral (green) and turn signal (red) lights, which were integrated into the stock Bullet speedo.